This invention relates generally to the design of a Transmit and Receive (TR) module, and, more specifically, to such Transmit and Receive modules suitable for a Phase Array Antenna (PAA) which could provide multiple simultaneous ground to satellite links with pointing and acquisition taking seconds. This invention also relates to the field of digital control design where a digital circuit is used to interface with the Antenna Control Computer to control the Transmit and Receive module.
Satellites require timely tracking, telemetry, and command (TT&C) for payload operation. The ground antenna is one of the key elements that enables satellite control and payload operations. To support the operation of a large number of satellites at various orbits, operators need a network of antennas distributed around the globe, such as the Air Force Satellite Control Network (AFSCN), to contact satellites at a predetermined time and location. Currently, they use large mechanically steered parabolic dishes to provide hemispherical coverage and simultaneous transmit (Tx) and receive (Rx) capabilities in support of Department of Defense (DoD) satellite operations (SATOPS). Network designers used reflector antennas because of relatively low acquisition cost. The current reflector antennas used to support satellite operations are approximately 10 m in diameter and are susceptible to single point failure and long downtime for repair and maintenance. The antenna can only link to one satellite at a time and must handle multiple satellite contacts serially. Because of the mechanical movement and heavy weight of the reflector antenna, operators cannot quickly schedule consecutive satellite contacts. The relatively long preparation and link time of reflector antennas produces a scheduled gap time of 30 minutes or more between two satellites. Because of these factors, the efficiency of reflector antenna operation is low in terms of throughput and turnaround time. The mechanical nature of the antenna also limits its flexibility to support new SATOPS requirements and operational concepts. In addition, the high operational and maintenance cost of a large reflector antenna contributes to its high life-cycle cost despite its lower initial cost. Other limitations include: cable wrap and keyhole effect. In addition, separate antennas are required for multiple satellite contacts. Current AFSCN resources are operating at or near saturation.
It is clearly desirable for the current satellite operations to have a more efficient and flexible antenna system. To date, phased array antennas have not been used for satellite TT&C operations primarily because of their high acquisition cost in comparison to technically inferior, but cheaper conventional reflector antennas. However, due to the maturation of S-band component technology provided by the cell phone industry, mass production of affordable electronically steered array (ESA) antennas is feasible. The electronically scanned phase array antenna (PAA) can offer superior performance, operability, adaptability, and maintainability for satellite operation.
Low cost component design and implementation issues are critical in developing a practical phased array antenna. Because the Transmit and Receive modules usually make up 40–50% of the PAA cost, it is very critical to minimize the T/R module cost and, consequently, the antenna cost.
Affordable phase antenna arrays operating at microwave frequencies are envisioned to consist of Transmit and Receive modules that employ microwave integrated circuits located at each radiating element of the aperture. The antenna system consists of separate receiver and transmit aperture capable of rapid beam motion. The transmitter antenna should be capable of high radiation power levels and the receiver antennas must achieve high G/T ratios. Beam agility and high-radiated power levels in association with the close spacing between the radiators drive the antenna design. The requirement for fast beam switching will require digital control circuits to calculate phase shift settings. A high RF radiated power level developed from closely spaced RF amplifiers generates very large heat densities. This forces the transmit antenna to increase in area to where beam pointing accuracy limits the array size. The great number of elements in the array emphasizes the need to develop a practical method of distributing control signals throughout the array. A Geodesic Spherical phase array antenna is considered for Air Force Satellite Communication network. Implicit in the system function array is the need to operate the array in full duplex operation. Additionally the array should be capable of controlling fundamental radiation characteristics such as bean width, beam size, side lobe levels and radiated power, in order to realize different antenna characteristics required by the various satellites. The array aperture consists of a large number of radiating elements that are spaced approximately half a wavelength at the upper end of the operational frequency band. The frequency response and excitation of each element in the aperture can be independently controlled. The aperture can be fully or partially utilized either to direct energy over a large volume or intentionally direct in a certain direction. Additionally, radar and communications require both transmission and reception of energy where as end system multicast (ESM) and Electronic Countermeasure (ECM) systems require only reception of energy. The capability of the array to provide transmit and receive functions simultaneous and to rapidly alter the set of configurations is possible due to active element digital control circuit. The active control circuits allow the Phase Array Radar to control their radiation characteristics. The aperture can be uniformly illuminated to achieve maximum gain or tapered illuminated to achieve low side lobes or shaped beam. The combination of the variable attenuator and phase shifter permits the array illumination to be modified and the antenna beam to be scanned in any direction. The filter specifies the portion of the aperture used by a particular system. The phase shifter, the variable attenuator and the amplifier are components that have been developed in MMIC, (microwave monolithic integrated circuit technology,) in the last decade.
Solutions are required to meet the prior art's need for a high degree of isolation between transmit and receive channels while maintaining the affordability associated with low-cost ceramic filters and traditional filters, low cost MMIC based power amplifiers for transmit channels and low-cost phase shifters.
Also needed are solutions, now lacking in the prior art, for interfacing a T/R module interface with a beam former, hot condition operation, polarization diversity, dual transmit and receive channels, low cost with justification, high isolation between transmit and receive channels, digital control on board, ruggedness and reliability.